These studies are designed to elucidate mechanisms associated with the proliferation and differentiation of normal red and white blood cell precursors in both experimental animals and man. In studies of erythropoiesis attention will be given to the developmental aspects of iron transport into erythroid cells. Other experiments will be concerned with the significance of the erythroid precursor cells, CFU-E and BFU-E, in normal and pathologic erythropoiesis. Factors which regulate normal granulocytopoiesis will be investigated with techniques of cell culture, including the diffusion chamber method and colony assays of hemopoietic precursor cells, with attention to the roles of the different types of stem cells, microenvironmental factors and cell-cell interactions. Similar methods will be used to investigate the proliferation of leukemic cells and the suppression of normal cell growth by acute leukemic cells. Additional studies will be concerned with the pathogenesis of neutropenia and drug-induced agranulocytosis.